Signal dependent self heating of circuit components in electronic amplifiers causes undesirable signal dependent thermally generated offset voltages having long time constants, which are associated with heat dissipation in the amplifier components. The time constants, or equivalently the frequency dependence, of the heat dissipation in the components is determined by the thermal properties of the components, which in turn are determined by geometry and material of the components.
In general a magnitude of the offset voltages is the strongest at DC (Direct Current) and will gradually taper off for frequencies above some low corner frequency. For example, in integrated bipolar junction transistor technology the bipolar junction transistors are the main sources of signal dependent thermally generated offset voltages. Frequency dependence of these offset voltages is typically flat from dc to a few hundred Hz and becoming insignificant above approximately 100 MHz.
The importance of such signal dependent thermally generated offset voltages is particularly shown in a step response of the wide band amplifier. The step response initially appears to settle in a few nanoseconds, but then slowly creeps towards a final value over several milliseconds. This type of problem is particularly troublesome in DC coupled, high gain, wide band amplifiers used in the vertical display channel in oscilloscopes, where such step settling is easily observed in display traces, over many decades of sweep speed of the oscilloscopes. Of course this type of problem is also troublesome in amplifiers used in other types of electronic instrumentation.
It is in general difficult to ensure stability--i.e. freedom from oscillation--in a wide band high gain amplifier. This is particularly so if a large amount of negative feedback is used to flatten the response of such an amplifier.
What is needed is a method and apparatus that substantially compensates for the signal dependent thermally generated offset voltages without introducing stability problems in the wide band high gain amplifier.